This invention relates to a method and apparatus for improving the formability of sheet metal and particularly for its use as a structural member. The prior art is clearly shown by the patent to Robb, U.S. Pat. No. 3,227,598 wherein a process using die pins is disclosed which enables formation of a high-strength, three-dimensional article from a single flat sheet of ductile material. Other patents also disclosing the use of die pins to deform sheet material are U.S. Pat. No. 3,525,663 and U.S. Pat. No. 3,902,348.
It has been found that a limiting factor experienced during the forming of unperforated sheet metal by the dies shown in the above-identified patents is the material thinning which occurred in the sheet metal in the region of the edge of the free end of the die pins. As the projections of a given sheet metal are increased in heighth greater stress is placed at this region. This thinning increases the likelihood of premature rupturing of the sheet. It is noted that U.S. Pat. No. 3,902,348 sets forth another method to attempt to overcome this shortcoming of the prior art method.